A possible reason why the Na+ ions added by the addion command do not
"follow rigouroulsy the phosphate backbone" may be due to the nature of
your model. For a "regular-backbone nucleic acids" you may indeed see
such behavior. However, if your oligo model is not regular, you should
expect to see deviations. Indeed, when I solvated a RNA-protein complex,
the Na+ ions did not rigorously follow the backbone.